A solution casting method is widely adopted in a production of film for optical use such as liquid crystal display (LCD) film, organic electroluminescence display (organic EL display) film, polarizer film, and a film for photosensitive material. The solution casting method is as follows. A dope obtained by dissolving cellulose acetate into a solvent is cast as a casting bead onto a support, and the support is moved to form a casting film. After being solidified, the casting film is peeled from the support by a peeling roller. While being transported by use of a roller and a tenter, the peeled casting film is dried to form a film.
According to the solution casting method, when a solution is cast onto the support at a high speed, vibration occurs on the casting bead by influence of air moving in accordance with the high-speed casting (accompanied air). In view of the above, a decompression chamber is disposed at the upstream side from the casting bead in a moving direction of the support such that the pressure in an area at the upstream side is lower than that in an area at the downstream side. Thereby, the accompanied air is prevented from being applied to the casting bead directly. However, regardless of such a countermeasure, air pressure vibration in the decompression chamber, air pressure vibration by a suction duct for decompression, wind pressure vibration such as vibration of the decompression chamber to which vibration of a blower is transmitted, and vibration disturbance by machine vibration cause the casting bead to vibrate. Therefore, minute stepped thickness unevenness periodically occurs in the casting direction of the film, thus causing a problem.
To solve the above problem, there is known a solution casting method (for example, Japanese Patent Laid-Open Publication No. 2001-18241 corresponding to U.S. Pat. No. 6,368,534), in which even if vibration occurs on the casting bead when dope is cast onto the support from the casting die, it is possible to decrease thickness unevenness of the film by controlling the length of the casting bead within the range of 3 mm to 40 mm.
However, in accordance with image definition enhancement of flat panel display (FPD) in recent years, a level of quality tolerance of thickness unevenness becomes high. Therefore, it becomes impossible to achieve sufficient quality by use of the solution casting method disclosed in Japanese Patent Laid-Open Publication No. 2001-18241. Accordingly, a solution casting method and apparatus capable of producing a film while further decreasing the thickness unevenness has been desired.